There is a system in which medical images captured by medical institutions are stored in a data center, and the stored medical images are shared between the medical institutions when the medical institutions access the data center. The terminal provided in the medical institution communicates with a server in real time through a network such as the Internet, and diagnosis may be performed so that a medical image that have been obtained by the communication is caused to be displayed on the terminal provided in the medical institution. As described above, any medical institution having a permission to access the medical images may utilize the medical images. That is, in the system, the medical images are shared between the medical institutions.
The pixel value of each pixel of the medical image indicates an X-ray absorption coefficient. The range of an X-ray absorption coefficient is different depending on a living tissue such as water, an adipose tissue, a soft tissue, or a bone, and the range of an X-ray absorption coefficient corresponding to each interpretation target is defined by a proportion of the living tissues such as water, an adipose tissue, a soft tissue, and a bone that constitute brain, heart, lung or the like that is the interpretation target of the medical image, so that a pixel value range corresponding to the interpretation target is defined based on the defined range of the X-ray absorption coefficient.
FIG. 1 is a diagram illustrating an example of pixel value ranges corresponding to interpretation targets. Parts A to D represent interpretation targets. An absolute value of each of the pixel value ranges and a relative relationship between the ranges are merely examples, but the part A is, for example, an example of a pixel value range of brain, and the part B is, for example, an example of a pixel value range of heart, the part C is, for example, an example of a pixel value range of lung, and the part D is, for example, an example of a pixel value range of liver.
Typically, a data amount of a medical image is very large. For example, the medical image has about 9,000 pixels in each of the vertical and horizontal directions. Recently, the medical image is displayed with a color depth of 12 to 16 bits, and each of the about 9,000 pixels in each of the vertical and horizontal directions has data corresponding to the color depth, so that a data amount of each of the medical images becomes very large.
As described above, the medical image has a large data amount, so that the data of the medical image is compressed and transmitted to a terminal. As an image encoding scheme used for the compression, for example, there are JPEG, MPEG-2, H.264/AVC, H.265/HEVC, and the like.
In addition, there is a case in which a medical image is displayed on a terminal so that a pixel value range is specified for the medical image. The color depth of the medical image corresponds to 12 to 16 bits, but a color depth of an image, which is allowed to be displayed on a typical display, is 8 bits, so that it is difficult to perform display of the whole pixel value range of the medical image on the terminal as is. Therefore, in this case, in the pixel value range of the medical image, a pixel value range having a certain range is displayed so as to match the color depth of the display so that mapping is performed. The pixel value range having the certain range is, for example, a pixel value range corresponding to an interpretation target.
Here, the mapping is processing in which the pixel value corresponding to the color depth of the medical image is associated with the pixel value corresponding to the color depth of the display, and as a scheme of the mapping, for example, there is a scheme regulated by Digital Imaging and Communication in Medicine (DICOM) that is a standard in which a communication protocol between medical imaging devices is defined.
Such a specification of the pixel value range is performed each time an interpretation target is changed. In addition, even in the same interpretation target, a pixel value range is changed each time an instruction is issued that specifies a pixel value range to make it easier to see the image. That is, each time a pixel value range is changed, an image based on the changed pixel value range is transmitted to the terminal.
A technology by which whether the pixel value of each pixel of image data exists within a certain range is determined, and high precision encoding is performed merely on pixels within the certain range, and low precision encoding is performed on pixels outside the certain range is discussed in Japanese Laid-open Patent Publication No. 2005-252556. In addition, a technology by which an image is encoded under different encoding conditions between a specified area and an area other than the specified area in the image is discussed in Japanese Laid-open Patent Publication No. 8-205144.